Radiation therapy is known in the art and serves a variety of medical purposes. Though some therapies involve exposing a patient's entire body to radiation (for example, when preparing a patient to receive a bone marrow transplant), in most cases the intent is to limit the radiation dosing to only specific areas of the patient's body (often referred to as a treatment volume). Various beam-limiting and/or beam-shaping devices are used for this purpose.
Such devices are located between the source and the patient; in some instances, they may be positioned very close to the source of radiation (e.g., primary collimator) while in other cases, they may be placed very close to the patient. The latter are sometimes referred to as patient collimators or blocks (hereinafter, the expression “block” will be used to refer to such a patient-proximal device). These blocks are made of a material (such as lead) that will efficiently block the radiation beam, save for an area or areas corresponding to one or more apertures formed through the material, to permit the beam to pass therethrough.
Ideally, such a block will frame the treatment volume in a manner that exposes only portions of the patient's body that require radiation treatment while preventing such exposure for other portions of the patient's body. The various treatment plans created for a given patient are evaluated on the basis of their success in delivering the prescribed dose to the target (and minimizing the dose in the surrounding healthy tissue); in this context, the correct shape and placement of the various collimators are typically of crucial importance.
The aperture design comprises a first-order effect with respect to the efficacy of a given radiation therapy treatment plan. There are, however, other potentially important contributing factors that have been (so far) overlooked in clinical applications. The fact that the block has a non-zero thickness is one such factor (as typical prior art planning reduces the block to a two-dimensional object (by disregarding its thickness) for computational simplicity). Another such factor is scattering of the beam off the material of the block. Though the fact that such effects induce higher-order influences, they may nevertheless induce sizable corrections to the local dose (e.g., to the dose delivered outside the target or at its margins).
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.